The present invention relates to a mirror smudge detecting apparatus used in a copier, and more particularly relates to an apparatus which detects, based on an amount of light reflected on a reference density section and entered into the apparatus through a mirror, a smudge on the mirror on which the light coming from a document is reflected.
A copier having the following structure is widely known as a generally used copier: a document placed on a transparent glass is illuminated from a lower portion; an optical image reflected from the document is reflected by a mirror so that the image is formed on a photoreceptor drum; the image is developed on the photoreceptor drum as a toner image; and after that, the image is transferred onto a copy sheet and fixed thereon (refer to Japanese Patent Publication Open to Public Inspection No. 82068/1989, and the like).
In the copier structured as described above, when a mirror, by which an optical image of the document is reflected, is smudged by toner or the like, a reflection ratio of the mirror is lowered, density for copying becomes higher, and the copying sheet becomes blackish, which is a problem.
Due to the foregoing, it is desired to provide an apparatus by which the mirror smudge is detected. However, it costs a great deal to exclusively provide a sensor for detecting the mirror smudge. Accordingly, the following apparatus has been previously proposed: a document density sensor, which is conventionally provided, is used for automatic density adjustment by which density of a copying sheet to be copied is automatically adjusted according to the density of the document; an amount of reflected light on the reference density section (a reference white plate) is detected by the document density sensor; and it is detected that an amount of reflected light from the reference density section is lowered due to the mirror smudge (refer to Japanese Patent Application No. 187142/1992).
However, in a document density detection system by the document density sensor, the system is set in the manner that the system can read out a broad range of the amount of light in order to cope with various document densities. Therefore, when the document density detection system is used in the conventional way, it can not highly accurately detect slight lowering of the amount of light caused by a mirror smudge, which is a problem. Further, in order to highly accurately detect the mirror smudge, it is necessary to adjust an output processing system of the sensor (an amplifier circuit), (gain adjustment, offset adjustment) so that it can cope with deviations of the document density sensor. In this case, it is troublesome for a serviceman to carry out the foregoing adjustment using a tester, which is a problem.
In view of the foregoing problems, the object of the present invention is to improve accuracy for the mirror smudge detection using the document density sensor, and to automatically carry out the adjusting operation to secure the foregoing accuracy.